Purification of mutacin III from group III Streptococcus mutans UA787 and genetic analyses of mutacin III biosynthesis genes

Purification and functional studies of a potent modified quorum-sensing peptide and a two-peptide bacteriocin in Streptococcus mutans

Bacteria use quorum-sensing signals or autoinducers to communicate. The signals in Gram-positive bacteria are often peptides activated by proteolytic removal of an N-terminal leader sequence. While investigating stimulation of antimicrobial peptide production by the Streptococcus mutans synthetic competence stimulating peptide signal (21-CSP), we found a peptide similar to the 21-CSP, but lacking the three C-terminal amino acid residues (18-CSP). The 18-CSP was more potent in inducing competence, biofilm formation, and antimicrobial activity than the 21-CSP. Our results indicate that cleavage of the three C-terminal residues occurred post export, and was not regulated by the CSP-signalling system. Deletion of comD encoding the CSP receptor abolished the competence and biofilm responses to the 21-CSP and the 18-CSP, suggesting that signal transduction via the ComD receptor is involved in the responses to both CSPs. In S. mutans GS5, beside the 18-CSP we also purified to homogeneity a two-peptide bacteriocin which production was stimulated by the 18-CSP and the 21-CSP. Partial sequence of the two-peptide bacteriocin revealed the product of the smbAB genes recently described. We found that the peptide SmbB was slightly different from the deduced sequence, and confirmed the prediction that both peptides constituting SmbAB bacteriocin are post-translationally modified. SmbAB exhibited antimicrobial activity against 11 species of streptococci, Enterococcus faecalis and Staphylocococcus epidermidis. Taken together, the findings support the involvement of the CSP response in bacteriocin production by streptococci and suggest a novel strategy to potentiate autoinducer activity.

Cell density- and ComE-dependent expression of a group of mutacin and mutacin-like genes in Streptococcus mutans

Streptococcus mutans is a major cariogenic inhabitant of the high cell density oral biofilm (dental plaque). In previous studies, we showed that production of one of its virulence factors, the bacteriocin mutacin IV, was regulated by high cell density as well as the competence regulatory system ComED. In this study, we utilized luciferase fusions and real-time reverse transcriptase polymerase chain reaction (RT-PCR), to demonstrate that high cell density and ComED also regulate an uncharacterized group of mutacin and mutacin-like genes. Under high cell density or in the presence of externally added competence-stimulating peptide (CSP), gene expression increased 10- to 30-fold. Interestingly, high cell density was able to bypass the requirement for CSP addition. However, both cell density and CSP-dependent gene expression had a strict requirement for the ComE response regulator.

Dispensable genes and foreign DNA in Streptococcus mutans

A range of properties, including the ability to utilize various sugars, bind macromolecules and produce mutacins, are known to vary in their occurrence in different strains of Streptococcus mutans. In addition, insertion-sequence elements show a limited distribution and sequencing of the genome of S. mutans UA159 has revealed the presence of putative genomic islands of atypical base composition indicative of foreign DNA. PCR primers flanking regions suspected of having inserted DNA were designed on the basis of the genome sequence of S. mutans UA159 and used to explore variation in a collection of 39 strains isolated in various parts of the world over the last 40 years. Extensive differences between strains were detected, and similar insertion/deletion events appear to be present in the genomes of strains with very different origins. In two instances, insertion of foreign DNA appears to have displaced original S. mutans genes. Together with previous results on the occurrence of deletions in genes associated with sugar metabolism, the results indicate that S. mutans has a core genome and a dispensable genome, and that dispensable genes have become widely distributed through horizontal transfer.

The rggC locus, with a frameshift mutation, is involved in oxidative stress response by Streptococcus thermophilus

In Streptococcus thermophilus, the locus rggC contains a frameshift mutation and thus consists of two open reading frames (ORFs), rggC (1) and rggC (2), which encode proteins exhibiting similarity with the Rgg transcriptional regulator family. In this work, mutants showing a partial deletion of rggC (1) and rggC (2 )were constructed and their response to menadione, a superoxide-generating compound, was analysed. These mutants exhibited different behaviour to this oxidative stress compared with the wild-type strain. Analysis of this locus among 21 strains of S. thermophilus showed a polythymine tract length variability and a strain-dependant adenine residue could be found upstream of this repeat. This interstrain polymorphism supports evidence for the hypothesis that the rggC locus is phase variable.

Mutacin H-29B is identical to mutacin II (J-T8)

BACKGROUND

Streptococcus mutans produces bacteriocins named mutacins. Studies of mutacins have always been hampered by the difficulties in obtaining active liquid preparations of these substances. Some of them were found to be lantibiotics, defined as bacterial ribosomally synthesised lanthionine-containing peptides with antimicrobial activity. The goal of this study was to produce and characterize a new mutacin from S. mutans strain 29B, as it shows a promising activity spectrum against current human pathogens.

RESULTS

Mutacin H-29B, produced by S. mutans strain 29B, was purified by successive hydrophobic chromatography from a liquid preparation consisting of cheese whey permeate (6% w/v) supplemented with yeast extract (2%) and CaCO3 (1%). Edman degradation revealed 24 amino acids identical to those of mutacin II (also known as J-T8). The molecular mass of the purified peptide was evaluated at 3246.08 +/- 0.1 Da by MALDI-TOF MS.

CONCLUSION

A simple procedure for production and purification of mutacins along with its characterization is presented. Our results show that the amino acid sequence of mutacin H-29B is identical to the already known mutacin II (J-T8) over the first 24 residues. S. mutans strains of widely different origins may thus produce very similar bacteriocins.

Production of the lantibiotic salivaricin A and its variants by oral streptococci and use of a specific induction assay to detect their presence in human saliva

Salivaricin A (SalA), the first Streptococcus salivarius lantibiotic to be characterized, appears to be inhibitory to most Streptococcus pyogenes strains. A variant of the SalA structural gene (salA1) is present in more than 90% of S. pyogenes strains, but only strains of M serotype 4 and T pattern 4 produce the biologically active peptide. The present study identifies four additional variants (salA2 to salA5) of the SalA structural gene and demonstrates that each of the corresponding inhibitory peptides (SalA2 to SalA5) is produced in vitro. These variants appear to be similar to SalA and SalA1 in their inhibitory activity against Micrococcus luteus and in their ability to act as inducers of SalA production. It had previously been shown that S. pyogenes strain SF370 had a deletion (of approximately 2.5 kb) in the salM and salT genes of the salA1 locus. In the present study, several additional characteristic deletions within the salA1 loci were identified. S. pyogenes strains of the same M serotype all share the same salA1 locus structure. Since S. salivarius is a predominant member of the normal oral flora of healthy humans, strains producing anti-S. pyogenes lantibiotics, such as SalA, may have excellent potential for use as oral probiotics. In the present study, we have used a highly specific SalA induction system to directly detect the presence of SalA in the saliva of humans who either naturally harbor populations of SalA-producing S. salivarius or who have been colonized with the SalA2-producing probiotic S. salivarius K12.

Identification of genes associated with mutacin I production in Streptococcus mutans using random insertional mutagenesis

Streptococcus mutans is a major pathogen implicated in dental caries. Its virulence is enhanced by its ability to produce bacteriocins, called mutacins, which inhibit the growth of other Gram-positive bacteria. The goal of this study is to use a random insertional mutagenesis approach to search for genes that are associated with mutacin I production in the virulent strain UA140. A random insertional mutagenesis library consisting of 11,000 clones was constructed and screened for a mutacin-defective phenotype. Mutacin-defective clones were isolated, and their insertion sites were determined by PCR amplification or plasmid rescue followed by sequencing. A total of twenty-five unique genes were identified. These genes can be categorized into the following functional classes: two-component sensory systems, stress responses, energy metabolism and central cellular processes. Several conserved hypothetical proteins with unknown functions were also identified. These results suggest that mutacin I production is stringently controlled by diverse and complex regulatory pathways.

Streptococcus rattusstrain BHT produces both a class I two-component lantibiotic and a class II bacteriocin

Streptococcus rattus strain BHT is a species representative and strong bacteriocin producer. Here we report that S. rattus BHT produces two quite different types of bacteriocin activity, named BHT-A and BHT-B. The two bacteriocins were purified and analysed for activity and by MALDI-TOF mass spectrophotometry. BHT-A was found to be a variant of the two-component lantibiotic, Smb. BHT-B is a non-modified 5195Da peptide with some similarity to the tryptophan-rich Staphylococcus aureus bacteriocin, aureocin A53. Six S. rattus and two S. mutans strains were found to contain both the BHT-A and BHT-B genetic loci.

Bacteriocin (mutacin) production by Streptococcus mutans genome sequence reference strain UA159: elucidation of the antimicrobial repertoire by genetic dissection

Streptococcus mutans UA159, the genome sequence reference strain, exhibits nonlantibiotic mutacin activity. In this study, bioinformatic and mutational analyses were employed to demonstrate that the antimicrobial repertoire of strain UA159 includes mutacin IV (specified by the nlm locus) and a newly identified bacteriocin, mutacin V (encoded by SMU.1914c).

LuxS controls bacteriocin production in Streptococcus mutans through a novel regulatory component

The oral pathogen Streptococcus mutans employs a variety of mechanisms to maintain a competitive advantage over many other oral bacteria which occupy the same ecological niche. Production of the bacteriocin, mutacin I, is one such mechanism. However, little is known about the regulatory mechanisms associated with mutacin I production. Previous work has demonstrated that the production of mutacin I greatly increased with cell density. In this study, we found that high cell density also triggered high level mutacin I gene transcription. However, this response was abolished upon deletion of luxS. Further analysis using real-time reverse transcription polymerase chain reaction (RT-PCR) demonstrated that in the luxS mutant transcription of both the mutacin I structural gene mutA and the mutacin I transcriptional activator mutR was impaired. Through microarray analysis, a putative transcription repressor annotated as Smu1274 in the Los Alamos National Laboratory Oral Pathogens Sequence Database was identified, which was strongly induced in the luxS mutant. Characterization of Smu1274, which we referred to as irvA, suggested that it may act as an inducible repressor to suppress mutacin I gene expression. A luxS and irvA double mutant regained the ability to produce mutacin I; whereas a constitutive irvA-producing strain was impaired in mutacin I production. These findings reveal a novel regulatory pathway for mutacin I gene expression, which may provide clues to the regulatory mechanisms of other cellular functions regulated by luxS in S. mutans.

Identification of nlmTE, the locus encoding the ABC transport system required for export of nonlantibiotic mutacins in Streptococcus mutans

Streptococcus mutans UA159, the genome sequence reference strain, exhibits nonlantibiotic bacteriocin (mutacin) activity. In this study, we have combined bioinformatic and mutational analyses to identify the ABC transporter designated NlmTE, which is required for mutacin biogenesis in strain UA159 as well as in another mutacin producer, S. mutans N.

Detection of mutA genes in transmitted strains and nontransmitted strains of mutans streptococci

Our aim was to determine whether an isolate carrying one of the mutA genes was related to its ability to be transmitted from mother to her child. First, 200 mutans streptococci isolates were genotyped by arbitrarily primed polymerase chain reaction (AP-PCR) to demonstrate transmission between 20 mother-child pairs and to detect the transmitted and nontransmitted strains. Then the mutacin structural genes mutA encoding mutacin types I, II, and III were screened by PCR. The results showed that all strains found to carry the mutAI gene were nontransmitted strains; PCR screening primers mutAII and mutAIII did not yield amplicons in any of the strains tested. Our data suggest that an isolate carrying the mutAI gene is related to reduced transmission. The low frequency of detection of mutAII, and mutAIII suggests that there is a high heterogeneity in the genetic determinants needed for the production of mutacin-like substances.

Frequency of four different mutacin genes in Streptococcus mutans genotypes isolated from caries-free and caries-active individuals

The ability of Streptococcus mutans to produce mutacins, combined with the production of other virulence factors such as lactic acid, may contribute to the pathogenesis of this bacterium. In the present study, the detection of genes encoding mutacin types I/III, II and IV was performed by PCR with specific primers to each type in a total of 63 S. mutans genotypes isolated from caries-active and caries-free individuals. In the caries-free group, PCR screening for mutacin IV revealed that 31.8% of strains were positive for this mutacin. PCR for the other three mutacins tested (I/III and II) did not yield amplicons in any S. mutans strains in this group. The PCR with primers of mutacin IV showed 68.3% positive genotypes in the caries-active group, on the other hand, the amplicons of mutacins I/III revealed 41.5% positive strains that carried these genes. The chi square test showed significant differences in the number of positive strains to mutacin IV when comparing the caries-free and caries-active genotypes of S. mutans (P = 0.01). All tested S. mutans strains were negative by PCR for mutacin II. The low frequencies of detection of some mutacin genes suggest the existence of high diversity and polymorphism in the production of genetic determinants of mutacin-like substances. In addition, the production of a wide spectrum of mutacins can play an important biological role in colonization by S. mutans strains, mainly in the niche of high-complexity microbial communities.

Genetic analysis of a unique bacteriocin, Smb, produced by Streptococcus mutans GS5

A dipeptide lantibiotic, named Smb, in Streptococcus mutans GS5 was characterized by molecular genetic approaches. The Smb biosynthesis gene locus is encoded by a 9.5-kb region of chromosomal DNA and consists of seven genes in the order smbM1, -T, -F, -M2, -G, -A, -B. This operon is not present in some other strains of S. mutans, including strain UA159. The genes encoding Smb were identified as smbA and smbB. Inactivation of smbM1, smbA, or smbB attenuated the inhibition of the growth of the indicator strain RP66, confirming an essential role for these genes in Smb expression. Mature Smb likely consists of the 30-amino-acid SmbA together with the 32-amino-acid SmbB. SmbA exhibited similarity with the mature lantibiotic lacticinA2 from Lactococcus lactis, while SmbB was similar to the mersacidin-like peptides from Bacillus halodurans and L. lactis. We also demonstrated that Smb expression is induced by the competence-stimulating peptide (CSP) and that a com box-like sequence is located in the smb promoter region. These results suggest that Smb belongs to the class I bacteriocin family, and its expression is dependent on CSP-induced quorum sensing.

A unique nine-gene comY operon in Streptococcus mutans

Many Gram-positive and Gram-negative bacteria possess natural competence mechanisms for DNA capture and internalization. In Bacillus subtilis, natural competence is absolutely dependent upon the presence of a seven-gene operon known as the comG operon (comGA-G). In species of Streptococcus, this function has been described for a four-gene operon (comYA-D in Streptococcus gordonii and cglA-D in Streptococcus pneumoniae). In this study, a nine-orf operon (named comYA-I) required for natural competence in Streptococcus mutans was identified and characterized. Orf analysis of this operon indicates that the first four Orfs (ComYA-D) share strong homology with ComYA-D of S. gordonii and CglA-D of S. pneumoniae, the fifth to seventh Orfs (ComYE-G) match conserved hypothetical proteins from various species of Streptococcus with ComYF possessing a predicted ComGF domain, the eighth Orf (ComYH) shows a strong homology to numerous DNA methyltransferases from restriction/modification systems, and the ninth Orf (ComYI) is homologous to acetate kinase (AckA). RT-PCR analysis of the orf junctions confirmed that all nine orfs were present in a single transcript, while real-time RT-PCR analysis demonstrated that these orfs were expressed at a level very similar to that of the first orf in the operon. Mutations were constructed in all nine putative orfs. The first seven genes (comYA-G) were found to be essential for natural competence, while comYH and comYI had reduced and normal natural competence ability, respectively. Analyses of S. mutans comY-luciferase reporter fusions indicated that comY expression is growth-phase dependent, with maximal expression at an OD(600) of about 0.2, while mutations in ciaH, comC and luxS reduced the level of comY expression. In addition, comY operon expression appears to be correlated with natural competence ability.

Interactions between oral bacteria: inhibition of Streptococcus mutans bacteriocin production by Streptococcus gordonii

Streptococcus mutans has been recognized as an important etiological agent in human dental caries. Some strains of S. mutans also produce bacteriocins. In this study, we sought to demonstrate that bacteriocin production by S. mutans strains GS5 and BM71 was mediated by quorum sensing, which is dependent on a competence-stimulating peptide (CSP) signaling system encoded by the com genes. We also demonstrated that interactions with some other oral streptococci interfered with S. mutans bacteriocin production both in broth and in biofilms. The inhibition of S. mutans bacteriocin production by oral bacteria was stronger in biofilms than in broth. Using transposon Tn916 mutagenesis, we identified a gene (sgc; named for Streptococcus gordonii challisin) responsible for the inhibition of S. mutans bacteriocin production by S. gordonii Challis. Interruption of the sgc gene in S. gordonii Challis resulted in attenuated inhibition of S. mutans bacteriocin production. The supernatant fluids from the sgc mutant did not inactivate the exogenous S. mutans CSP as did those from the parent strain Challis. S. gordonii Challis did not inactivate bacteriocin produced by S. mutans GS5. Because S. mutans uses quorum sensing to regulate virulence, strategies designed to interfere with these signaling systems may have broad applicability for biological control of this caries-causing organism.

Improved methods for mutacin detection and production

Studies of mutacins have always been hampered by the difficulties in obtaining active liquid preparations of these substances. In order to be commercially produced, good mutacin yields have to be obtained, preferably in inexpensive media. The results presented here indicate that mutacins can be produced in supplemented cheese whey permeate. The influence of carbon and nitrogen supplements on mutacin production varied according to the producer strain. The use of CaCO3 as a buffer in batch cultures resulted in improved yields of mutacin in the supernatants. Antimicrobial activity assays were improve by acidification of the diluent (pH 2) and were less variable in peptone water (0.5%). The culture medium consisting of cheese whey permeate (6% w/v), yeast extract (2% w/v) and CaCO3 (1% w/v) was found to be an inexpensive medium for the efficient production of mutacins.

Inactivation of the ciaH Gene in Streptococcus mutans diminishes mutacin production and competence development, alters sucrose-dependent biofilm formation, and reduces stress tolerance

Many clinical isolates of Streptococcus mutans produce peptide antibiotics called mutacins. Mutacin production may play an important role in the ecology of S. mutans in dental plaque. In this study, inactivation of a histidine kinase gene, ciaH, abolished mutacin production. Surprisingly, the same mutation also diminished competence development, stress tolerance, and sucrose-dependent biofilm formation.

Bovicin HJ50, a novel lantibiotic produced by Streptococcus bovis HJ50

A bacteriocin-producing strain was isolated from raw milk and named Streptococcus bovis HJ50. Like most bacteriocins produced by lactic acid bacteria, bovicin HJ50 showed a narrow range of inhibiting activity. It was sensitive to trypsin, subtilisin and proteinase K. Bovicin HJ50 was extracted by n-propanol and purified by SP Sepharose Fast Flow, followed by Phenyl Superose and Sephadex G-50. Treatment of Micrococcus flavus NCIB8166 with bovicin HJ50 revealed potassium efflux from inside the cell in a concentration-dependent manner. The molecular mass of bovicin HJ50 was determined to be 3428.3 Da. MS analysis of DTT-treated bovicin HJ50 suggested that bovicin HJ50 contains a disulfide bridge. The structural gene of bovicin HJ50 was cloned by nested PCR based on its N-terminal amino acid sequence. Sequence analysis showed that it encodes a 58 aa prepeptide consisting of an N-terminal leader sequence of 25 aa and a C-terminal propeptide domain of 33 aa. Bovicin HJ50 shows similarity to type AII lantibiotics. Chemical modification using an ethanethiol-containing reaction mixture showed that two Thr residues are modified.

Determination of mutacin activity and detection of mutA genes in Streptococcus mutans genotypes from caries-free and caries-active children

Relationships between genetic diversity, mutacin production and sensitivity to mutacins in Streptococcus mutans were evaluated in 19 clinical isolates from caries-free and caries-active children. Mutacin production was tested against 30 indicator strains; results showed significant variations in the inhibitory spectra of the clinical isolates. There was no association between the inhibitory spectrum of the infecting strain and the caries experience or the level of mutans streptococci infection of the host. Homology to the mutA gene coding for mutacin II was detected in one clinical isolate; none of the clinical isolates showed homology to the mutA genes coding for mutacins I or III. Genotyping by random amplified polymorphic DNA (RAPD) reactions grouped the isolates into three clusters, but no correlation was found between any of the clusters and mutacin activity, caries experience or level of mutans streptococci in the host.

Mutation of luxS affects biofilm formation in Streptococcus mutans

Quorum sensing is a bacterial mechanism for regulating gene expression in response to changes in population density. Many bacteria are capable of acyl-homoserine lactone-based or peptide-based intraspecies quorum sensing and luxS-dependent interspecies quorum sensing. While there is good evidence about the involvement of intraspecies quorum sensing in bacterial biofilm, little is known about the role of luxS in biofilm formation. In this study, we report for the first time that luxS-dependent quorum sensing is involved in biofilm formation of Streptococcus mutans. S. mutans is a major cariogenic bacterium in the multispecies bacterial biofilm commonly known as dental plaque. An ortholog of luxS for S. mutans was identified using the data available in the S. mutans genome project (http://www.genome.ou.edu/smutans.html). Using an assay developed for the detection of the LuxS-associated quorum sensing signal autoinducer 2 (AI-2), it was demonstrated that this ortholog was able to complement the luxS negative phenotype of Escherichia coli DH5alpha. It was also shown that AI-2 is indeed produced by S. mutans. AI-2 production is maximal during mid- to late-log growth in batch culture. Mutant strains devoid of the luxS gene were constructed and found to be defective in producing the AI-2 signal. There are also marked phenotypic differences between the wild type and the luxS mutants. Microscopic analysis of in vitro-grown biofilm structure revealed that the luxS mutant biofilms adopted a much more granular appearance, rather than the relatively smooth, confluent layer normally seen in the wild type. These results suggest that LuxS-dependent signal may play an important role in biofilm formation of S. mutans.

Macedocin, a food-grade lantibiotic produced by Streptococcus macedonicus ACA-DC 198

Streptococcus macedonicus ACA-DC 198, a strain isolated from Greek Kasseri cheese, produces a food-grade lantibiotic named macedocin. Macedocin has a molecular mass of 2,794.76 +/- 0.42 Da, as determined by electrospray mass spectrometry. Partial N-terminal sequence analysis revealed 22 amino acid residues that correspond with the amino acid sequence of the lantibiotics SA-FF22 and SA-M49, both of which were isolated from the pathogen Streptococcus pyogenes. Macedocin inhibits a broad spectrum of lactic acid bacteria, as well as several food spoilage and pathogenic bacteria, including Clostridium tyrobutyricum. It displays a bactericidal effect towards the most sensitive indicator strain, Lactobacillus sakei subsp. sakei LMG 13558(T), while the producer strain itself displays autoinhibition when it is grown under conditions that do not favor bacteriocin production. Macedocin is active at pHs between 4.0 and 9.0, and it retains activity even after incubation for 20 min at 121 degrees C with 1 atm of overpressure. Inhibition of macedocin by proteolytic enzymes is variable.

Different bacteriocin activities of Streptococcus mutans reflect distinct phylogenetic lineages

Bacteriocins produced by mutans streptococci are known as mutacins. In this study 16 broadly active mutacin-producing Streptococcus mutans strains from New Zealand, North America and Europe were classified into four groups (A-D) on the basis of differences in their activity in deferred antagonism tests against either the homologous producer strain (to test for presence of self-immunity) or indicator strains Staphylococcus aureus 46 and Enterococcus faecium TE1. Two of the strains included in the study (UA140 and UA96) were representatives of the group I and II mutacin producer strains previously described by Caufield and co-workers. One of the New Zealand isolates of group A (S. mutans strain N) appeared to produce inhibitory activity similar to that of the group I prototype strain UA140. Four other New Zealand isolates of group B (S. mutans strains M19, M34, B34 and D14) had mutacin II-like activity. The group B mutacin producers differed from the group A mutacin producers in their additional activity against Staph. aureus 46. Seven S. mutans strains (M46, B46, B57, M12, M28, B28 and 13M) were distinguished from the group A and group B mutacin producers in that they inhibited E. faecium TE1. These were called group C mutacin producers. Strains H7 and H23 resembled the group C strains in their action on both indicator strains TE1 and 46. However, these two strains failed to exhibit immunity to their own inhibitory products in the deferred antagonism test and were separately classified as group D mutacin producers. Phylogenetic analysis of the strains by several genotypic and phenotypic characteristics revealed that the mutacin groups were associated with distinct evolutionary lineages of S. mutans.

LasX, a transcriptional regulator of the lactocin S biosynthetic genes in Lactobacillus sakei L45, acts both as an activator and a repressor

The 11 kb las locus, present on the 50 kb plasmid pCIM1, specifies the production of the lantibiotic lactocin S in Lactobacillus sakei L45. The gene cluster is organized into two oppositely orientated operons, lasAMNTUVPJW (lasA-W) and lasXY, the former of which contains the biosynthetic, immunity and transport genes. We have previously shown that inactivation of lasX abolishes lactocin S production and causes a drastic reduction in lasA-specific transcripts (encoding pre-lactocin S). The aim of this study was to determine whether or not the product of lasX, which is significantly similar to Rgg-like regulators, was directly involved in transcriptional regulation of the lactocin S biosynthetic genes. The divergently orientated and overlapping promoters, P(lasA)(-W) and P(lasXY), were transcriptionally fused to the Escherichia coli gusA gene, and the activity of the fusions was assayed in the presence and absence of lasX, which was expressed on a separate plasmid. A significant stimulation of expression (5-6-fold) of the P(lasA-W)-gusA fusion was observed in the presence of lasX, whereas expression of the P(lasXY)-gusA construct was reduced 1.5-2-fold. Our results strongly suggest that LasX is a bifunctional regulatory protein, acting both as an activator of lasA-W transcription and as a repressor of lasXY transcription. While a transcription stimulation activity has been described for several of the Rgg-like proteins, the present study is the first to report an autorepressor function for a member of this protein group.

The flavoprotein MrsD catalyzes the oxidative decarboxylation reaction involved in formation of the peptidoglycan biosynthesis inhibitor mersacidin

The lantibiotic mersacidin inhibits peptidoglycan biosynthesis by binding to the peptidoglycan precursor lipid II. Mersacidin contains an unsaturated thioether bridge, which is proposed to be synthesized by posttranslational modifications of threonine residue +15 and the COOH-terminal cysteine residue of the mersacidin precursor peptide MrsA. We show that the flavoprotein MrsD catalyzes the oxidative decarboxylation of the COOH-terminal cysteine residue of MrsA to an aminoenethiol residue. MrsD belongs to the recently described family of homo-oligomeric flavin-containing Cys decarboxylases (i.e., the HFCD protein family). Members of this protein family include the bacterial Dfp proteins (which are involved in coenzyme A biosynthesis), eukaryotic salt tolerance proteins, and further oxidative decarboxylases such as EpiD. In contrast to EpiD and Dfp, MrsD is a FAD and not an FMN-dependent flavoprotein. HFCD enzymes are characterized by a conserved His residue which is part of the active site. Exchange of this His residue for Asn led to inactivation of MrsD. The lantibiotic-synthesizing enzymes EpiD and MrsD have different substrate specificities.

Identification, characterization, and expression of a second, bicistronic, operon involved in the production of lactocin S in Lactobacillus sakei L45

Through the analysis of spontaneous insertion mutants of Lactobacillus sakei L45, a second operon involved in lactocin S production was identified and characterized. The new, bicistronic unit, termed lasXY, is situated immediately upstream of the previously characterized nine-open reading frame (ORF) lactocin S operon (lasA-W) and is transcribed in the opposite direction. The proximal of the two newly identified genes, lasX, specifies a 285-residue protein that is similar to a group of proteins with reported gene regulation functions in gram-positive bacteria. It was demonstrated that the lasX mutants have a strongly reduced level of lasA and lasA-W mRNA, thus indicating the likely cause of the Bac(-) phenotype of these mutants. The second ORF in the operon, lasY, specifies a 300-residue ABC transporter homolog, the function of which is currently obscure. Transcription initiation mapping of the lasXY operon demonstrates that the two lactocin S promoters overlap such that both transcripts initiate within the -35 region of the oppositely oriented promoter. This organization of promoters is unique among this group of regulons and may constitute a modulatory site in the proposed LasX-dependent expression of lasA and downstream genes.

Diversity of Streptococcus mutans bacteriocins as confirmed by DNA analysis using specific molecular probes

Mutacin-producing strains have been classified into 24 groups (designated by letters A to X) by similarity in activity spectra and cross-immunity. Similarity in primary structure among these groups can be revealed using DNA hybridization. The amino acid sequences of four mutacins (B-Ny266, 1140/mutacin III and mutacin II) were used to design two DNA probes in order to detect similar genes among groups of Streptococcus mutans strains demonstrating inhibitory activity. In addition to the appropriate parent strain, each probe hybridized with the total DNA from only two out of the 24 mutacin group type strains. Thus, the remaining 18 groups of strains produce mutacins that differ from the mutacins sequenced to date. In order to explore the similarity between genes coding for mutacins B-Ny266 and JH1140, the group B specific probe was utilized to detect a DNA fragment of 1.9 kb in the genome of S. mutans strain Ny266. The sequence of the cloned fragment codes for three open reading frames (lanA, lanA' and lanB) similar to those of strains JH1140 and UA787. The gene lanA' is strongly similar to the structural gene lanA (67%), but only one RNA transcript of about 300 bases was detected by Northern hybridization using the lanA-lanA' probe. Transcription of lanA alone was verified by RT-PCR.

The group I strain of Streptococcus mutans, UA140, produces both the lantibiotic mutacin I and a nonlantibiotic bacteriocin, mutacin IV

Strains of Streptococcus mutans produce at least three mutacins, I, II, and III. Mutacin II is a member of subgroup AII in the lantibiotic family of bacteriocins, and mutacins I and III belong to subgroup AI in the lantibiotic family. In this report, we characterize two mutacins produced by UA140, a group I strain of S. mutans. One is identical to the lantibiotic mutacin I produced by strain CH43 (F. Qi et al., Appl. Environ. Microbiol. 66:3221-3229, 2000); the other is a nonlantibiotic bacteriocin, which we named mutacin IV. Mutacin IV belongs to the two-peptide, nonlantibiotic family of bacteriocins produced by gram-positive bacteria. Peptide A, encoded by gene nlmA, is 44 amino acids (aa) in size and has a molecular mass of 4,169 Da; peptide B, encoded by nlmB, is 49 aa in size and has a molecular mass of 4,826 Da. Both peptides derive from prepeptides with glycines at positions -2 and -1 relative to the processing site. Production of mutacins I and IV by UA140 appears to be regulated by different mechanisms under different physiological conditions. The significance of producing two mutacins by one strain under different conditions and the implication of this property in terms of the ecology of S. mutans in the oral cavity are discussed.

Covalent structure of mutacin 1140 and a novel method for the rapid identification of lantibiotics

The primary structure of the Streptococcus mutans lantibiotic mutacin 1140 was elucidated by NMR spectroscopy, mass spectrometry, and chemical sequencing. The structure is in agreement with other closely related lantibiotics, such as epidermin. A novel method was developed in which mutacin 1140 was chemically modified with sodium borohydride followed by ethanethiol, allowing the differentiation of the thioether-containing residues from the dehydrated residues. This double-labeling strategy provides a simple method to reliably identify all modified lantibiotic residues with a minimal amount of material. While NMR spectroscopy is still required to obtain thioether bridging patterns and thus the complete covalent structure, the double-labeling technique, along with mass spectrometry, provides most of the information in a fraction of the time required for a complete NMR analysis. Thus, with these new techniques lantibiotics can be rapidly characterized.

Crystal structure of the peptidyl-cysteine decarboxylase EpiD complexed with a pentapeptide substrate

Epidermin from Staphylococcus epidermidis Tü3298 is an antimicrobial peptide of the lantibiotic family that contains, amongst other unusual amino acids, S:-[(Z:)- 2-aminovinyl]-D-cysteine. This residue is introduced by post-translational modification of the ribosomally synthesized precursor EpiA. Modification starts with the oxidative decarboxylation of its C-terminal cysteine by the flavoprotein EpiD generating a reactive (Z:)-enethiol intermediate. We have determined the crystal structures of EpiD and EpiD H67N in complex with the substrate pentapeptide DSYTC at 2.5 A resolution. Rossmann-type monomers build up a dodecamer of 23 point symmetry with trimers disposed at the vertices of a tetrahedron. Oligomer formation is essential for binding of flavin mononucleotide and substrate, which is buried by an otherwise disordered substrate recognition clamp. A pocket for the tyrosine residue of the substrate peptide is formed by an induced fit mechanism. The substrate contacts flavin mononucleotide only via Cys-Sgamma, suggesting its oxidation as the initial step. A thioaldehyde intermediate could undergo spontaneous decarboxylation. The unusual substrate recognition mode and the type of chemical reaction performed provide insight into a novel family of flavoproteins.

Posttranslationally modified bacteriocins--the lantibiotics

Lantibiotics are a subgroup of bacteriocins that are characterized by the presence of the unusual thioether amino acids lanthionine and 3-methyllanthionine generated through posttranslational modification. The biosynthesis of lantibiotics follows a defined pathway comprising modifications of the prepeptide, proteolytic activation, and export. The genes encoding the biosynthesis apparatus and the lantibiotic prepeptide are organized in clusters, which also include information for proteins involved in regulation and producer self-protection. The elongated cationic lantibiotics primarily act through the formation of pores and recent progress with nisin and epidermin has shown that specific docking molecules such as lipid II play an essential role in this mechanism. Mersacidin and actagardine inhibit cell wall biosynthesis by complexing the precursor lipid II, whereas the cinnamycin-like peptides bind to phosphoethanolamine thus inhibiting phospholipase A2.

Molecular characterization of lantibiotic-synthesizing enzyme EpiD reveals a function for bacterial Dfp proteins in coenzyme A biosynthesis

The lantibiotic-synthesizing flavoprotein EpiD catalyzes the oxidative decarboxylation of peptidylcysteines to peptidyl-aminoenethiols. The sequence motif responsible for flavin coenzyme binding and enzyme activity is conserved in different proteins from all kingdoms of life. Dfp proteins of eubacteria and archaebacteria and salt tolerance proteins of yeasts and plants belong to this new family of flavoproteins. The enzymatic function of all these proteins was not known, but our experiments suggested that they catalyze a similar reaction like EpiD and/or may have similar substrates and are homododecameric flavoproteins. We demonstrate that the N-terminal domain of the Escherichia coli Dfp protein catalyzes the decarboxylation of (R)-4'-phospho-N-pantothenoylcysteine to 4'-phosphopantetheine. This reaction is essential for coenzyme A biosynthesis.

Purification and biochemical characterization of mutacin I from the group I strain of Streptococcus mutans, CH43, and genetic analysis of mutacin I biosynthesis genes

Previously, we reported isolation and characterization of mutacin III and genetic analysis of mutacin III biosynthesis genes from the group III strain of Streptococcus mutans, UA787 (F. Qi, P. Chen, and P. W. Caufield, Appl. Environ. Microbiol. 65:3880-3887, 1999). During the same process of isolating the mutacin III structural gene, we also cloned the structural gene for mutacin I. In this report, we present purification and biochemical characterization of mutacin I from the group I strain CH43 and compare mutacin I and mutacin III biosynthesis genes. The mutacin I biosynthesis gene locus consists of 14 genes in the order mutR, -A, -A', -B, -C, -D, -P, -T, -F, -E, -G, orfX, orfY, orfZ. mutA is the structural gene for mutacin I, while mutA' is not required for mutacin I activity. DNA and protein sequence analysis revealed that mutacins I and III are homologous to each other, possibly arising from a common ancestor. The mature mutacin I is 24 amino acids in size and has a molecular mass of 2, 364 Da. Ethanethiol modification and peptide sequencing of mutacin I revealed that it contains six dehydrated serines, four of which are probably involved with thioether bridge formation. Comparison of the primary sequence of mutacin I with that of mutacin III and epidermin suggests that mutacin I likely has the same bridging pattern as epidermin.

Biosynthesis of the lantibiotic mersacidin: organization of a type B lantibiotic gene cluster

The biosynthetic gene cluster (12.3 kb) of mersacidin, a lanthionine-containing antimicrobial peptide, is located on the chromosome of the producer, Bacillus sp. strain HIL Y-85,54728 in a region that corresponds to 348 degrees on the chromosome of Bacillus subtilis 168. It consists of 10 open reading frames and contains, in addition to the previously described mersacidin structural gene mrsA (G. Bierbaum, H. Brötz, K.-P. Koller, and H.-G. Sahl, FEMS Microbiol. Lett. 127:121-126, 1995), two genes, mrsM and mrsD, coding for enzymes involved in posttranslational modification of the prepeptide; one gene, mrsT, coding for a transporter with an associated protease domain; and three genes, mrsF, mrsG, and mrsE, encoding a group B ABC transporter that could be involved in producer self-protection. Additionally, three regulatory genes are part of the gene cluster, i.e., mrsR2 and mrsK2, which encode a two-component regulatory system which seems to be necessary for the transcription of the mrsFGE operon, and mrsR1, which encodes a protein with similarity to response regulators. Transcription of mrsA sets in at early stationary phase (between 8 and 16 h of culture).